You need to go through all the levels of the seventh board and do all the objectives. Here is a video made by myself showing you how to handle the different missions.
If you only need a few levels, you can watch the specific video guide below.
Level 7-1 :
Level 7-2 :
Level 7-3 :
Level 7-4 :
Level 7-5 :
Level 7-6 :
Level 7-7 :
Level 7-8 :